OSPF Areas: Normal, Stub, and NSSA

OSPF Area Types

• OSPF uses different area types to optimize routing and manage large-scale networks: Normal Areas, Stub Areas, and Not-So-Stubby Areas (NSSA).

Normal Areas

• Supports all OSPF route types, including intra-area, inter-area, and external routes (Type 5 LSAs).
• Receives all LSA types (Type 1, 2, 3, 4, 5).
• Interfaces with other areas and can import/export routes to/from other OSPF areas and external networks.
• Typically used for core and main areas where full routing information is needed.
• Pros: full visibility of the entire OSPF network, complete routing information for optimal path selection.
• Cons: higher memory and CPU usage, more complex to manage.

Stub Areas

• Does not accept external routes (Type 5 LSAs), instead, a default route is injected by the ABR.
• Receives only Type 1, 2, and 3 LSAs.
• Summarize external routes and inject a default route into the stub area.
• Ideal for areas where external connectivity is limited and a simple routing structure is required.
• Pros: reduced LSA database size, lower CPU and memory requirements, simplified configuration and maintenance.
• Cons: limited visibility of external networks, may result in suboptimal routing.

Not-So-Stubby Areas (NSSA)

• Allows limited external routes (Type 7 LSAs) that can be converted to Type 5 LSAs by the ABR.
• Receives Type 1, 2, 3, and 7 LSAs, but not Type 5 LSAs.
• Convert Type 7 LSAs to Type 5 LSAs for external areas.
• Useful for areas that need to inject external routes without fully supporting all external LSAs.
• Pros: flexibility to introduce external routes into a semi-stub area, reduced overhead compared to normal areas.
• Cons: complexity in managing and converting Type 7 to Type 5 LSAs, may still have higher memory and CPU usage compared to stub areas.

Choosing the Right OSPF Area

• Depends on specific network requirements, including need for external routing information, resource constraints, and desired level of routing complexity.